The thermoplastic elastomer composition of the present invention comprises a thermoplastic elastomer which includes an ethylene•α-olefin copolymer and a crystalline polyethylene resin, wherein the crystalline polyethylene resin constitutes a three-dimensional network structure in a matrix formed by the ethylene•α-olefin copolymer, an organic or inorganic blowing agent and a nucleating agent, wherein the thermoplastic elastomer has a melt flow rate of 5 g/10 min or more at a temperature of 230° C. and a load of 10 kg, and a melt tension of 3.0 gf or more at a temperature of 210° C. and a pulling rate of 2 m/min. Also the foam injection molding method of the present invention is a method including injecting the above-mentioned composition into a cavity space of a metal mold and then expanding the cavity space to foam by opening the metal mold at a mold opening rate of 0.05 to 0.4 mm/sec.